The present invention relates to a system for extracting sludge from a circular decanter of the type currently used in liquid treatment plants, and particularly as used in sewage water purification plants. The system of the present invention is particularly applicable to circular decanters of the rotary bridge type, wherein sludge is sucked from the bottom of the tank into a receptacle or trough which is integral with the rotary bridge.
Rotary bridge decanters are known wherein the rotary bridge is supported by a stationary hollow central column, liquid to be treated is supplied through the hollow column to the decanter, a deflector is positioned concentrically around the hollow column for uniformly distributing the liquid to be treated, and sludge to be evacuated from the bottom of the decanter is evacuated through the hollow column.
In known decanters of this type however, certain inherent constructional and operational problems exist. Specifically, in such known decanters it has up to now been difficult to provide for transfer of evacuated sludge from the trough into the central column. Such sludge transfer is normally performed either by the operation of siphons or by simple gravity flow, but in either case difficult construction problems attended by serious limitations in treatment efficiency have been encountered.
The most popular attempt to solve these problems has involved the utilization of a movable siphon attached to the rotary bridge and adapted to direct the sludge into a fixed circular trough adjacent the central hollow column. This sludge is subsequently discharged from this circular trough by overflow. However, the presence of this trough makes it necessary to locate the input deflector for the distribution of the liquid to be treated at a relatively low level in the decanter in order to obtain a desired water flow rate. The liquid to be treated is deaerated only after its passage beneath the deflector, and this detrimentally affects the efficiency of the overall treatment operation which requires a thorough and complete deaeration of the liquid.
Another known arrangement employs a movable annular spout externally adjacent the input deflector, such spout being rigidly attached to the bridge to rotate therewith. The sludge flows from the collecting trough through sludge suction conduits into the movable annular spout, and the sludge is discharged from the annular spout into the central column. It will be apparent that the construction of this type of arrangement is complicated. Furthermore, it has been found to be difficult to properly adjust the rate of suction of the sludge.
Another known system provides for the sludge to flow outwardly from the collecting trough to a movable spout which is fixed to the bridge, the sludge then being discharged by gravity flow into a discharge conduit within the central column. Rotary mechanical seals are necessary to seal the spout to the discharge conduit. The proper adjustment of these seals, particularly due to their inherent large dimensions, is difficult. The provision of such spout also is attended by difficulty in obtaining satisfactory input distribution and deaeration of the liquid to be treated.
In all of the above mentioned known systems, inherent operational problems are due to the apparent necessity of an intermediate movable or fixed annular channel or spout.